1. Field of the Invention
The present invention relates to a method of manufacturing spatial light modulator and electronic device employing it.
2. Description of Prior Art
This type of spatial light modulator is disclosed, for example Japanese Patent Application Laid-Open Nos. 4-230722, 5-188308, and 5-196880. An improved form of these devices is also described in the March 1994 issue of "Nikkei Microdevice" as a "DMD" (Digital Micromirror Device).
This DMD has, as shown in FIG. 22, a three-layer construction comprising an upper layer 800, an intermediate layer 810, and a lower layer 830.
The upper layer 800 comprises a mirror 802 and a mirror support post 804 joined to the center of the lower surface of the mirror 802. In connection with the fabrication process of the mirror 802, in a position opposite to the mirror support post 804 is formed a depression 806.
The intermediate layer 810 has a mirror support plate 812 which is coupled to the mirror support post 804, and which is supported at opposite ends by hinges 814 so as to be able to be driven in an inclining manner. To provide the space for this mirror support plate 812 for driving in an inclining manner, the hinges 814 have on their lower sides hinge support posts 816.
The intermediate layer 810 further is provided with first and second address electrodes 818 and 820 on opposing sides of the hinges 814, each supported by electrode supporting posts 826. Furthermore, outside this are provided a first mirror contact electrode 822 and second mirror contact electrode 824, each supported by electrode supporting posts 826.
The lower layer 830 comprises four electrodes 832a to 832d coupled to the electrode supporting posts 826 of the first and second address electrodes 818 and 820, and a common electrode 834 coupled to the first and second mirror contact electrodes 822 and 824.
This DMD, as shown in FIG. 23, has a bias voltage Va applied to the mirror 802 and the first and second mirror contact electrodes 822 and 824. Then when for example a negative voltage is applied to the first address electrode 818, and a positive voltage is applied to the second address electrode 820, a Coulomb force acts between the mirror 802 and the first address electrode 818, and the mirror 802 is driven to an inclined position as shown by the dot-dash line in FIG. 23. By reversing the polarity of the voltage applied to the first and second address electrodes 818 and 820, an inclined position as shown by the dot-dot-dash line in FIG. 23 can be established.
The inclined position of the mirror 802 shown by a dot-dash line in FIG. 23 is taken to be the "ON" position in which light is reflected toward a certain position, and the inclined position shown by a dot-dot-dash line is taken to be the "OFF" position in which light is reflected in a different direction. By varying the time between switches, a 256-gradation display can be obtained.
The DMD shown in FIG. 22 is hypothetically manufacturable by a fabrication process as shown in FIGS. 24A to 24H and FIGS. 25A to 25F. FIGS. 24A to 24H show the steps in the formation of the intermediate layer 810 on an already formed lower layer 830, and FIGS. 25A to 25F show the steps in the formation of the upper layer 800 on the intermediate layer 810, and the formation of the interlayer spaces.
As shown in FIG. 24A, a substrate 840 on which an SRAM (static random access memory) is formed as the lower layer 830 is provided. Next, as shown in FIG. 24B, a resist 842 is coated on this substrate 840, and in the stage shown in FIG. 24C a pattern corresponding to the hinge support posts 816 and electrode supporting posts 826 is formed.
As shown in FIG. 24D, an aluminum (Al) film is formed by vapor deposition over the surface of the resist 842 and trench portion, and then further as shown in FIG. 24E an aluminum oxide film 846 is formed over the surface.
Further after vapor deposition of an aluminum film 848 as shown in FIG. 24F, as shown in FIG. 24G a resist 850 is applied in a pattern. Thereafter, as shown in FIG. 24H, the aluminum film 848 is etched, whereby mirror support plate 812, hinges 814, and hinge support posts 816 are formed.
By the process shown in FIGS. 25A to 25F, the upper layer 800 shown in FIG. 22 is formed. For this purpose, as shown in FIG. 25A a resist 852 is applied thickly, and is formed in a pattern as shown in FIG. 25B. Further, an aluminum film 854 is formed by vapor deposition, and after an aluminum oxide film 856 is formed over a part of the surface thereof, the extremities of the aluminum film 854 are removed by etching, whereby the mirror 802 and mirror support post 804 are formed. (See FIGS. 25C to 25E.)
Finally, as shown in FIG. 25F, by removing the resist 842 and 852, a space between the upper layer 800 and intermediate layer 810 is formed, and moreover a space between the intermediate layer 810 and lower layer 830 is formed.
However, in the above process, there is the problem that the DMD cannot be obtained with a high yield. One reason for this is that the factor determining the angle of inclination of the mirror 802, that is, the distance between the lower surface of the mirror 802 and the mirror contact electrodes 822 and 824 depends on the thickness of the resist 852 in the resist step shown in FIG. 25A.
In general, such a resist is formed by the spin coating method, and while it is difficult in itself to improve the uniformity of a resist layer thickness, when the spin coating method is used it is extremely difficult to make the resist 852 of a uniform thickness.
Moreover, in the conventional spin coating method, the larger the surface area of the wafer, the more difficult it is to ensure uniformity within the area of the resist film, and further to make the thickness of the resist film constant is for a large diameter semiconductor wafer almost impossible. Thus, it is difficult to form a plurality of devices simultaneously from a single semiconductor wafer, and the throughput is reduced.
In addition to the above problems, a further one is that in the stage of removing the resist shown in FIG. 25F, it is difficult to completely remove the resist from the furthest recesses of the underside of the mirror 802 and hinges 814. If foreign objects are thus left behind, the mirror 802 and address electrodes 818 and 820 may short-circuit, or the inclination of the mirror may be obstructed, or the mirror contact electrodes 822 and 824 and address electrodes 818 and 820 may short-circuit.
Another problem with the above described construction of a DMD is that the depression 806 is formed in the center region of the mirror 802. In the aluminum vapor deposition step of FIG. 25C, when aluminum is vapor deposited in the trench portion, the position opposing this trench is inevitably concave, and the forming of the depression 806 cannot be prevented.
In this three-layer DMD, since the hinges 814 are not in the same plane as the mirror 802, the exposed surface area of the mirror 802 is increased, and the benefit is obtained of an increased light utilization ratio.
However, since the depression 806 is formed in the center of the large area mirror 802, with this depression 806 in the line of a powerful beam of light, the light utilization ratio is actually reduced by the diffuse reflection. Alternatively, the diffusely reflected light may be input as information pertaining to another pixel, resulting in the problem of reduced image quality. Moreover, even if the side walls of the depression 806 are processed so as to be vertical, the area which is optically effective is reduced.
A further problem is that the above described spatial light modulator is formed on a substrate 840 on which an SRAM is formed, and the overall yield is the product of the yield of the SRAM and the yield of the spatial light modulator, which is thus considerably low.
Another prior art is the spatial light modulator described in Petersen, "Silicon as a Mechanical Material.Arrow-up bold. Proceedings of the IEEE, Vol. 70, No. 5, May 1982, in FIGS. 39, 40 and 41 on pages 442 and 448. In order to fabricate this, a silicon substrate which has been cut and ground on both sides is used, and a micromirror is formed on this silicon substrate by photolithography and etching processes. The silicon substrate on which this micromirror is formed and a glass plate on which a metal electrode film is formed are bonded by the anode bonding method, and a spatial light modulator thus manufactured.
By this method, however, in order to cut and grind the silicon substrate on both sides, and thus determine the substrate thickness, it is not possible to obtain a thickness less than 200 .mu.m. This is because grinding to a thickness less than this leads to breakage of the silicon substrate. The thickness of the micromirror is therefore at least 200 .mu.m, and the inertial moment due to this heavy mass is thus great, making rapid response and high resolution display impossible.